A vehicle sensor is responsive to acceleration and/or deceleration or tilting of a vehicle above a predetermined level. The sensor is incorporated into a traditional seat belt retractor and operates to displace a lever that engages a ratchet wheel and locks the retractor against further payout of seat belt webbing. The vehicle occupant is thereby secured against further forward motion and preventing injury by impact with internal parts of the vehicle.
Known vehicle sensors comprise an inertial mass that is displaced in its mounting by a sudden deceleration of the vehicle indicating that a crash is imminent or in progress. The mass moves a lever which is brought onto engagement with teeth on a ratchet wheel fixed to the web-winding spool of the retractor. The ratchet wheel and thus the spool are thus locked against further rotation, the seat belt webbing is prevented from further protraction and the vehicle occupant is secured.
One such known inertial mass comprises a ball resting in a hollow mount. The lever rests on the ball and lifts when the ball moves to ride up a side of the hollow. However, such a sensor requires precision engineering to obtain the required accurate tolerances and tends to exhibit undesirable friction and noise problems. The ball requires accurate machining and grinding and the production costs are high.
Another known form of sensor uses a so-called "standing man" in place of the ball. The standing man comprises a hollow lead mass mounted on a plastic cup surrounding an upstanding pin. The lever rests on the pin, which sits in sensor housing. A sudden deceleration causes the mass to tilt and this in turn lifts the pin, which lifts the lever and locks the retractor. Such a "standing man" sensor is well known to skilled people working in the field. It is a complicated assembly and thus expensive to manufacture. In addition there is an undesirably high level of friction and this degrades its sensitivity. In particular, a different angle of tilt is observed for the point at which the lever is lifted sufficiently to lock the retractor compared to the angle of tilt required to release the lever and unlock the retractor.
Another undesirable characteristic of such known sensors is that the lever is lifted by different degrees depending on the direction of the activating acceleration, since it depends on the direction in which the mass tilts. The lever tends to lift higher if the mass tilts towards the pivot point of the lever than if the mass tilts away from the pivot point. This makes the sensor more difficult to set accurately before installation in the vehicle because the calibration of the lever position is dependent on whether the retractor is to be placed on the right or on the left of a vehicle. This right/left dependency increases production costs. A substantially constant mechanical advantage would be preferable.
According to the present invention there is provided a vehicle sensor for a vehicle safety restraint comprising a sensor mass in the form of an upturned hollow cup resting on an upstanding post, and a lever resting on the cup, wherein the sensor mass is formed in one piece.
The sensor mass may be formed, for example, by injection molding or die casting or by any other method with which the skilled man would be familiar. It may be of metal or of a high-density plastic material.
According to one embodiment of the present invention the side of the lever which contacts the mass has a profiled portion. For example, a profiled pad may be molded or affixed to the underside of the lever. Alternatively the upper surface of the cup may be suitably profiled. A profile which tapers towards the lever pivot point would be one example, but a stepped profile would be another example. The relative profiles of the lever and the cooperating surface of the mass are arranged so that the lever lifts through substantially the same angle whichever way the mass is tilted and is thus independent of the direction of acceleration activating the sensor. This means that the sensor is relatively independent of its positioning in the vehicle and also results in better repeatability and improved consistency and accuracy in its performance.
In one embodiment the pad is affixed off-center relative to the center axis of the mass, and extends further on the side of the center axis away from the pivot point of the lever, than on the side towards the pivot point.
In addition the arrangement is such that the effect of friction is significantly reduced by minimizing the movement arm for each friction force acting on the sensor, and by minimizing the number of friction contact points.
The mass is formed in one piece and this may, for example, be by injection molding or by die-casting. It is considerably cheaper to manufacture than the traditional standing man type sensor mass, which comprises three different parts, or the ball type, which requires very accurate machining and grinding. Thus improvements in performance are realized with the present invention together with a cost reduction.
According to a preferred embodiment of the present invention the geometry of the hollow inside the mass is arranged so that the mass is constrained in its movement on the post so that it does not come into contact with any other component of the assembly (other than the lever). Thus degradation of the performance due to friction is reduced and also rattling noise is reduced.
The mass is preferably shaped so that its center of mass is located as far as possible from its center of rotation. This minimizes the effect of friction forces on the sensitivity of the sensor. A bell shape with a particularly bulbous skirt bottom edge is advantageous.